Which vacuum cleaner should I buy?

I can't decide between the Dyson Animal, the Shark Navigator or the Hoover (where the middle pops out), or something else. I'm looking for something that is functional for doing stairs. I'm concerned about paying $600 for a Dyson, if the Shark Navigator is just as good for half the price. I have 3 cats, so it being above average for picking up hair is essential. If a person could answer who has one, or has used any of these options, it would be appreciated. Serious answers only, please.